Atrioventricular Septal Defect − AV Canal

Atrioventricular septal defects (AVSD) are a relatively common family of congenital heart defects.

Also known as atrioventricular canal defects or endocardial cushion defects, they account for about 5 percent of all congenital heart disease, and are most common in infants with Down syndrome. (About 15 percent to 20 percent of newborns with Down syndrome have atrioventricular septal defects).

The primary defect is the failure of formation of the part of the heart that arises from an embryonic structure called the endocardial cushions. The endocardial cushions are responsible for separating the central parts of the heart near the tricuspid and mitral valves (AV valves), which separate the atria from the ventricles.

The structures that develop from the endocardial cushions include the lower part of the atrial septum (wall that divides the right atrium from the left atrium) and the ventricular septum (wall that divides the right ventricle from the left ventricle) just below the tricuspid and mitral valves.

The endocardial cushions also complete the separation of the mitral and tricuspid valves by dividing the single valve between the embryonic atria and ventricles. An atrioventricular septal defect may involve failure of formation of any or all of these structures.

Most commonly, atrioventricular septal defects can be classified into one of three categories called complete, partial (or incomplete), or transitional.

A complete atrioventricular septal defect is one in which there are defects in all structures formed by the endocardial cushions. Therefore, there are defects (holes) in the atrial and ventricular septa, and the AV valve remains undivided or "common."

A partial or incomplete atrioventricular septal defect is one in which the part of the ventricular septum formed by the endocardial cushions has filled in, either by tissue from the AV valves or directly from the endocardial cushion tissue, and the tricuspid and mitral valves are divided into two distinct valves.

The defect is, therefore, primarily in the atrial septum and mitral valve. This type of atrial septal defect is referred to as an ostium primum atrial septal defect, and is usually associated with a cleft in the mitral valve that may cause the valve to leak.

The transitional type of defect looks similar to the complete form of atrioventricular septal defect, but the leaflets of the common AV valve are stuck to the ventricular septum thereby effectively dividing the valve into two valves and closing most of the hole between the ventricles.

As a result of the effectively small defect between the ventricles, a transitional atrioventricular septal defect behaves more like a partial atrioventricular septal defect, even though it looks more like a complete atrioventricular septal defect.

Atrioventricular septal defect is also a common part of more complex heart disease that occurs in heterotaxy syndromes.

These forms of atrioventricular septal defects usually result in the common AV valve opening predominantly into only one of the ventricles with the other ventricle being underdeveloped. These situations are more accurately described as single ventricle lesions such as hypoplastic left heart syndrome or tricuspid atresia.

Atrioventricular septal defects can also occur with other types of congenital heart disease such as coarctation of the aorta or tetralogy of Fallot.

The specific type of defect strongly influences the symptoms that may develop, along with the timing and details of surgical repair.

A complete atrioventricular septal defect allows oxygenated blood that has returned from the lungs to the left atrium and ventricle to cross either the atrial or ventricular septum and go back out the pulmonary artery to the lungs.

This re-circulation of blood to the lungs, called a left-to-right shunt, is inefficient because the left ventricle must pump a volume of already oxygenated blood back to the lungs while trying to meet the body's usual demand for its own oxygenated blood.

The amount of extra blood pumped by the left ventricle is often an additional two to three times that required of a left ventricle in an anatomically normal heart.

Because there is a large hole in the ventricular septum, the high pressure normally generated by the left ventricle to propel blood throughout the body is also transmitted to the lungs. Under normal circumstances, the lungs have a blood pressure much lower than that in the rest of the body.

The presence of a large left-to-right shunt, the associated increased workload on the left ventricle, and the high pulmonary artery pressure cause the lungs to become engorged with blood, and causes fluid to leak from the bloodstream into the air spaces of the lungs.

This condition is called pulmonary edema and makes it harder for a baby with this condition to move his or her lungs and breathe comfortably. The combination of increased heart and lung work uses large amounts of calories and results in the constellation of symptoms referred to as congestive heart failure (CHF).

Babies with congestive heart failure breathe fast and hard, often sweat and / or tire out while feeding, and grow slowly or sometimes even lose weight. These symptoms usually develop gradually over the first one to two months of life.

The doctor will usually hear a heart murmur when this type of defect is present. The murmur is caused by the blood passing from the left ventricle to the right ventricle and out the pulmonary artery.

A small number of infants with a complete atrioventricular septal defect will not develop congestive heart failure. This occurs because in some cases, the muscle cells that line the small arteries to the lungs get bigger and constrict to try to protect the lungs from the extra flow and high pressure caused by the atrioventricular septal defect.

Called increased pulmonary vascular resistance (PVR) or pulmonary vascular disease, this condition is more common in infants with Down syndrome.

The increase in pulmonary vascular resistance is very effective in preventing the signs and symptoms of congestive heart failure by minimizing the amount of left-to-right shunt, and may even cause blood with low oxygen to go from the right ventricle to the left ventricle and out to the body without picking up oxygen.

This causes cyanosis, which is a bluish discoloration of the skin, fingernails and mouth and it may also cause the murmur to be softer.

While infants with a complete atrioventricular septal defect and elevated pulmonary vascular resistance often grow better and appear healthier than those with low pulmonary vascular resistance and congestive heart failure, the occurrence of increased pulmonary vascular resistance may prompt early surgical correction of the defect.

Repair of the atrioventricular septal defect lowers the pressure in the pulmonary artery and allows these muscles to relax before they become permanently constricted.

Infants with the partial or transitional forms of atrioventricular septal defects have more subtle signs and symptoms. Like children with a complete atrioventricular septal defect, they have an increased volume of blood passing through the pulmonary artery.

The main difference between a left-to-right shunt that occurs primarily between the atria rather than the ventricles is that the pressure in the pulmonary artery usually remains low despite the increase in flow.

This causes less work for the heart and lungs and results in fewer breathing and growth problems. It also lessens the possibility that the pulmonary vascular resistance will increase.

Nevertheless, there is an increased workload on the heart and growth may occur more slowly than infants and children with normal hearts. There is usually a heart murmur present, but it is softer than that which occurs with a complete atrioventricular septal defect.

These types of defects may not come to medical attention until the child is several months or even years old because of the subtlety of the signs and symptoms that may be associated with them.

Significant congestive heart failure, growth failure or a very loud murmur in a child with a partial atrioventricular septal defect can occur when the defect in the mitral valve leaflet causes this valve to be very leaky.

A heart murmur is often the first clue that this heart defect exists. It is typically noted in the first week or two of life and it is not uncommon that no murmur is present at birth.

The diagnosis of atrioventricular septal defect in any form is easily made by echocardiography. Chest X-ray and an electrocardiogram may be used to assist in the assessment. Both may show characteristic findings in atrioventricular septal defects.

Because of the high incidence of atrioventricular septal defects in infants with Down syndrome, all infants with Down syndrome should have an echocardiogram, even if they do not have a heart murmur or any of the signs or symptoms discussed above.

Symptomatic infants with atrioventricular septal defects may improve with medicine, but in all cases corrective heart surgery will be necessary.

Medicines commonly used to treat congestive heart failure from left-to-right shunts in infants include diuretics such as Lasix (furosemide) and angiotensin converting enzyme (ACE) inhibitors such as enalapril.

These type of defects will never close on their own and will always require corrective surgery for treatment.

Medical treatment of infants with atrioventricular septal defects is usually used to relieve symptoms and allow the baby to get big enough to undergo surgical repair with lower risks.

This usually occurs at 3-6 months for infants with a complete atrioventricular septal defect and 6-18 months for infants with a partial atrioventricular septal defect.

Surgical repair of either type of defect involves closure of the holes in the atrial and / or ventricular septa with a patch or patches, and reconstruction of the common atrioventricular valve.

A particularly challenging aspect of the repair of a complete atrioventricular septal defect is dividing the common AV valve found in this condition.

Complications following surgery can arise if the opening in the mitral valve is now too narrow or it is still very leaky. Other problems to be avoided include narrowing the path for blood to pass from the left ventricle to the aorta, or disturbances of the electrical system of the heart.

The specialized tissue that conducts the impulse for the heart to beat runs very near the area where the stitches for the ventricular patch need to be placed. If this is disrupted, placement of a pacemaker may be necessary.  

Atrioventricular Septal Defects Treatment Outcomes

The usual recovery period following repair of a partial atrioventricular septal defect is relatively brief. Most patients are out of the intensive care unit (ICU) in one to two days and home in four to five days following surgery.

Reported surgical survival is greater than 97 percent but is probably close to 100 percent in the current era.

Repair of a complete atrioventricular septal defect is often more complex and may be associated with other factors that can prolong the postoperative course.

In particular, the presence of elevated PVR preoperatively can necessitate a prolonged time on a mechanical ventilator and the need for higher amounts of medication to help the heart work well after surgery.

Additionally, problems with the mitral valve being too leaky, the path out of the left ventricle being too narrow or with the electrical system of the heart are more common after this type of surgery.

Most patients require two to four days in the intensive care unit after repair of a complete atrioventricular septal defect, and a five- to seven-day hospital stay. Recent results indicated that operative survival is around 97 percent for this type of operation.

The most common later problem that is seen following surgery is a leaky mitral valve which may require reoperation in up to 10 percent of patients, but most become medication-free and are free of cardiac symptoms.  

Follow-up visits with the cardiologist are important to assess valve and heart muscle function and continued antibiotic prophylaxis for endocarditis is recommended.

Most adult patients with an AVSD have had surgical repair. Most continue to do very well for the rest of their lives, but patients should be monitored by a congenital heart expert periodically because of the potential problems of a leaky or narrow “mitral” valve, subaortic stenosis (obstructed blood flow to the body from the main pumping chamber), high blood pressure in the lungs (pulmonary arterial hypertension), and heart rhythm problems.  A small minority of patients will require reoperation as adults because of these problems.

AVSDs occur in so-called “partial” or “complete” forms.

Adult patients with a complete AVSD who have not been repaired almost always have pulmonary arterial hypertension and require careful management. Some patients with a partial AVSD may have the same problem.  The complete form of a VSD is most common in patients who have Down syndrome or so-called trisomy 21.

Adult patients with a partial AVSD or so-called ostium primum ASD will usually require surgical repair as adults if not repaired by that time. This is because the right heart chambers enlarge as for any type of atrial septal defect, and eventually problems will occur that will impair patients’ physical abilities and shorten their expected lifespans.

Learn more about the Adolescent and Adult Congenital Heart Disease Program.


Last Updated 11/2013